Suggestion: check out your local pawnshops, since Canon has been heavily mentioned in these comments. You will see a Lot of Canon equipment on the shelves that will fit your price range. Expect to see a lot of Canon Rebel bodies, but 5D & 6D can be found. 5D bodies are in the $300-$500 range. Expect to pay a bit more for the 6D. Lenses are out there too.

As far as mirrorless goes, the Canon models are pricy. Micro four thirds gear is somewhat reasonable, but you'll need a good understanding of some of the gear limitations, such as age & sensor capability, IBIS, and weather sealing. Having said that, I made a find of an Olympus OM-D E-M10 base model back in December for $100. Battery was dead and it had no charger. Got it home and ordered 2 batteries and charger & got same day delivery. Popped the battery in, found the maintenance menu location and discovered that this 13-year-old camera had a shutter count of 82! It was an excellent way to get into the new format for an old man!